1. Field of the Invention
The present invention relates to an exposure mechanism to be used for forming a hole when a semiconductor device is fabricated, and more particularly to an exposure mechanism having a hole pattern of which line width is hard to vary after development even if there is the focus variation of an exposure device, i.e., in the state of defocus such as the shift of a focal position of exposed light on a resist, for example, 1 or 2.mu.m focal shift when the exposed light is radiated onto the resist so as to form a desired resist pattern in a photolithography process in which the exposure device is used.
2. Description of the Prior Art
In general, there has been adopted the following exposure mechanism in order to form a resist in a wafer by means of the hole pattern of an exposure device so as to have a predetermined shape, and then form a hole such as a contact hole having a predetermined diameter by using a resist pattern as a mask.
As shown in FIG. 19,
(a) exposed light L is emitted from a mercury lamp 31 opposed to a converging mirror 30, passes through a pair of relay mirrors 32 and then reaches a condenser lens 33, and
(b) the exposed light passes through the condenser lens 33, a hole pattern 1 having a predetermined shape on a reticle and a reduction lens 34, and is converged by an autofocus optical system as a focusing mechanism including a light source 35 and a detector 36 such that its focal plane comes to the resist film of a water 38 provided on a wafer chuck 37.
FIGS. 20, 22 and 24 show the positional relationship between a wafer 38 comprised of a substrate 39 and a resist layer 12, and the focal plane of the exposed light passing through the holes pattern 1. The substrate 39 is comprised of a Si substrate on which a semiconductor device (not shown) is formed and an insulating film laminated on the Si substrate. The resist layer 12 is laminated over the substrate 39.
In FIGS. 20, 22 and 24, the reference designation of denotes one of focal points determined by the reduction lens 34 when the exposed light passing through the hole pattern 1 passes through the reduction lens 34.
In FIG. 20, a focal plane 13a is provided in the resist layer 12 (the state of just focus), for example. Consequently, there can be obtained a resist pattern 14a having a predetermined shape (see FIG. 21).
In FIG. 22, a focal plane 13b corresponds to the upper face of the resist layer 12 (the state of defocus), for example. Consequently, there cannot be obtained a resist pattern having a predetermined shape. More specifically, the focal plane 13b is shifted upward from the focal plane 13a so that light necessary for exposure does not reach the lower portion of the resist layer 12. Consequently, there is formed a resist pattern 14b having a resist portion which is not exposed (see FIG. 23).
In FIG. 24, a focal plane 13c corresponds to the lower face of the resist layer 12 (the state of defocus), for example. Consequently, there cannot be obtained a resist pattern having a predetermined shape. More specifically, the focal plane 13c is shifted downward from the focal plane 13a so that a resist portion to remain is exposed. Consequently, there is formed a resist pattern 14c in which the resist portion to remain is removed (see FIG. 25).
The focal position of exposed light which is incident on the resist film 12 is shifted for the following reasons.
1. The misrecognition of the upper and lower positions of the wafer 38 by the autofocus optical systems 35 and 36.
2. The flatness and slant of the wafer caused by film formation on a semiconductor device or the like in a semiconductor fabricating process.
3. The flatness of the wafer chuck 37 supporting the wafer 38.
4. The shift of a focal position in an exposed light radiation area which is caused by the aberration of a lens in an exposure mechanism (focal shift in the lens).
From the foregoing, there is determined focusing precision at an exposure step. Conventionally, when the focus of an exposure device is slightly shifted (up to 1.mu.m), a dimension after development is greatly varied. By way of example, the dimension after development is reduced to 1 to several .mu.m.
Referring to an exposure mechanism according to the prior art, there has been used a hole pattern having only one square or rectangular light transmitting portion through which exposed light can pass.
As shown in FIG. 26, a hole pattern 51 includes a square light transmitting portion (hole) 52 having a side L (line segment B'B=line segment BB"=L) of 3.6.mu.m and a light intercepting portion 53 which forms the light transmitting portion 52. Consequently, when the focus of the exposure device is slightly shifted (up to 1.mu.m), the dimension after development may greatly be varied. It is just the same with a hole pattern having a rectangular light transmitting portion.
FIG. 27 shows a curve a illustrating the change of relative light intensity distribution in the case where a quantity of focal shift is 0.mu.m, a curve b illustrating the change of relative light intensity distribution in the case where the quantity of focal shift is 1.mu.m, and a curve c illustrating the change of relative light intensity distribution in the case where the quantity of focal shift is 2.mu.m, in which the hole pattern 51 shown in FIG. 26 is used.
The relative light intensity distribution is calculated by computed simulation. In FIG. 27, an axis of ordinates denotes a relative light intensity. In FIG. 19, to 1 is set a point at which the focal plane of exposed light passing through the reduction lens 34 comes to the resist film of the wafer 38. An axis of abscissas denotes a distance from the center T of the hole pattern 51. An area surrounded by a broken line is a light transmitting portion 52. The edge of the hole pattern 51 is shown by a broken line apart from the center T by a distance of 1.8.mu.m as shown by an arrow E (In FIG. 26, the edge is shown by line segments TB and BB"). The line segment TB has length of 1.8.mu.m.
In the case where the hole pattern 51 shown in FIG. 26 according to the prior art is used, the curves a, b and c are changed over a relative light intensity of 0 to 1.1 when the quantity of focal shift is varied.
However, the change of a line width, i.e., developing shift S produces an effect on the diameter of a contact hole formed on the insulating layer of a wafer. When focusing variation is not caused as in case of just focus in which the quantity of focal shift is 0.mu.m as shown in FIG. 17, contact hole 20a formed on the insulating layer 20 of the wafer has a diameter P2. As shown in FIGS. 16 and 18, contact holes 21a and 22a formed on insulating layers 21 and 22 have diameters Q2 and R2, respectively. In general, the developing shift S is obtained by subtracting a mask dimension M (respresented by an opening diameter L in FIG. 26) from the hole diameter P2, Q2 and R2 after development.
In case of defocus in which the quantity of focal shift is 1 or 2.mu.m, it is seen that the diameters Q2 and R2 of the contact holes 21a and 22a formed on the insulating layers 21 and 22 are greatly shifted from the diameter P2 of the contact hole 20a shown in FIG. 17. More specifically, the shape of the contact hole in the state of defocus is shifted from that in the state of just focus.
It is an object of the present invention to provide an exposure mechanism in which a line width is hard to change even if the focus of an exposure device is varied.